Electron discharge device



H. E. KALLMANN ET AL ELECTRONDISCHARGE DEVICE Jan. 10, 1939.

Filed Aug. '13, 1956 2 Sheets-Sheet l ORNEY Jan. 10, 1939. H. E.KALLMANN ET AL v 2,143,378

ELECTRON DISCHARGE DEVICE Filed Aug. 13, 1936 2 Sheets-Sheet 2 I 7574 420 1 /4/ 1/1, I/ l/ INVENTORS HE/A/ZEQW/A/ MzLMAA A/ 41v 0770 $1 5M? 7 MMATTORNEY the production of a Patented Jan. 10, 1939 PATENT GFFECE2,143,378 ELECTRON DISCHARGE DEVICE Heinz Erwin Kallmann,

Klemperer, Iver, Engl & Musical Industries Limited,

Ealing, London, and Otto and, assignors to Electric Middiesex, England,a British company Application August 13, 1936, Serial No. 95,744 InGreat Britain August 21, 1935 10 Claims.

The present invention relates to electron dis charge devices.

It has been proposed to obtain electrical amplification by the so-calledmethod of electron mul tiplication in which use is made of the fact thatan electron impinging with suitable velocity on a solid body may causeseveral secondary electrons to be emitted. In a proposed arrangement forcarrying this method into eiiect two electrodes, at least one of whichis coated with photo-sensitive material, are placed opposite one anotherin an evacuated envelope. Also in the envelope and surrounding the spacebetween these two electrodes is arranged a tubular anode. A generatorof'high frequency electrical oscillations is associated with the firstmentioned two electrodes (which will hereinafter be referred to for conVenience as the oscillator electrodes) by means of which high frequencyalternating potentials are applied across them. The anode is connectedthrough an output impedance such as a resistance to the positiveterminal of a source of constant potential such as a battery, thenegative terminal of which is connected to the electrical centre of thegenerator. The device operates in the following way.

The input signals are in the form of light of varying intensity, andthis light is allowed to fall onto the photo-sensitive oscillatorelectrode. When this electrode is at a negative potential with respectto the oscillator electrode opposite, the photo-electrons liberated areaccelerated towards the impinge upon this opposite electrode, andliberate secondary electrons therefrom, which, if the impact velocity ofthe photo-electrons is suitably chosen, are greater in number than theincident photo-electrons. The frequency of the alternating potentialsapplied to the electrodes is so chosen that the potentials of theelectrodes are now reversed, so that the secondary electrons areaccelerated towards the photo-sensitive electrode, and the process ofproduction of secondary electrons will be repeated. Thus there will beproduced an oscillating cloud of electrons rapidly increasing indensity. The electrons may be prevented from spreading by suitablemagnetic field. The anode surrounding the space between the electrodeshas a positive potential, and has the efiect of drawing oif a certainfraction of the electrons from the space inside it. This current,passing through the output resistance gives rise to potentialdifferences across the latter, which potential difierences are used asthe output potentials of the device.

Means are also associated with the device whereby the oscillation isprevented from building up indefinitely, so that variations in the lightincident on the photo-sensitive electrode produce correspondingvariations in the output potential. 5 To this end the oscillation isperiodically stopped, for instance by stopping the generator ofoscillations, or by periodically applying to one of the oscillatorelectrodes .a positive or negative pulse. Thus if the generator producesan oscillation 0 having a frequency of 50 megacycles per second, thestopping pulse may recur at a frequency of 5 megacycles per second.

Now we have found that the range of potentials which may be applied tothe anode to pro- 15 duce satisfactory amplification lie between verynarrow limits (assuming the other factors to remain unchanged). Whencurrent flows in the anode circuit, the potential of the anode will bechanged, and this change of potential is sufficiently great to upset thesatisfactory working of the device, by causing changes in the currentoscillating between the electrodes and in the background current, thatis the current which is present in the device even when no light fallson 25 the photo-sensitive electrode.

We have also found that if a curve is plotted showing the potentialdistribution along the axis of the device (distance along the axis beingplotted as abscissa against potential as ordinate) the 30 curve soobtained is saddle shaped, rising as the oscillator electrodes areapproached and showing a minimum in the centre. The shape of this curvedetermines the time of flight of the electrons between the electrodes,and the slope of the 35 ends of the curve determines the fraction of theelectron cloud which is withdrawn by the anode. It will be seentherefore that variation in anode potential which will affect thepotential distribution along the axis of the device may cause the 40time of flight of electrons from one oscillator electrode to the otherto become diiferent from the half period of oscillation of the potentialon the oscillator electrodes, and may also adversely aifect theequilibrium conditions.

The invention may best be understood by referring to the drawings, inwhich:

Figure l is a diagram showing the relation between anode voltage andanode current in an electron multiplier.

Figure 2 shows one form of the invention, and

Figure 3 shows a modified or alternate form of the invention.

The manner in which changes in anode potential affect the working of thedevice described 55.

above may be seen from Fig. 1 of the accompanying drawings, which is adiagram representing the relation between anode voltage plotted along OXand anode current plotted along OY. The curve a shows the relationbetween anode cur rent and voltage when no light is allowed to fall onthe photo-electrically active oscillator electrode. It will be notedthat the device exhibits a negative resistance. The dotted curve bshowsthe corresponding relationship when light of given intensity is allowedto fall on the photoelectrically active oscillator electrode. It will benoted that curve b has a negative slope of smaller value than curve a.These curves are obtained experimentally in the usual manner.

Referring to Fig. 1, let it be assumed that the device is given avoltage at on its anode, and no light falls on the photo-electricallyactive oscillator electrode. The anode current will then be given by141. Now if light of a given intensity falls on the photo-electricallyactive oscillator electrode, in the absence of any anode loadresistance, the anode current will rise to a value 212. The presence ofan anode load resistance will however cause a drop in anode voltage dueto the increase in anode current from 1 1 to 1/2, and consequently thedevice will be working on a different portion of its characteristic. Theamount of movement along the characteristic will clearly depend on thechange in anode current, and hence on the intensity of light falling onthe photo-electrically active oscillator electrode. Since the curves aand b have different slopes, the output current will not be proportionalto the intensity of the incident light, and if the latter is a beam ofvarying intensity, a distortion of the variations will occur in theoutput of the device.

It is an object of the present invention to provide an improved electrondischarge device of the kind hereinbefore referred to, in which theeffect of variations in anode potential is diminished or removed.

According to the present invention, an electron multiplying arrangementcomprising an evacuated envelope, two oscillator electrodes arrangedwithin said envelope in spaced relation, means for applying alternatingpotential difierences between said electrodes for the purpose o1oscillating electrons between them, input means for varying the numberof electrons oscillating between said oscillator electrodes, an anodesur rounding or partly surrounding the space 'between said electrodes,and a circuit for maintaining said anode at a potential positive withrespect to the mean potential of said oscillator electrodes, so thatsaid anode is adapted to draw ofif some of said electrons, said circuitincluding an output impedance, there are provided means for diminishingor removing the efiect of changes of potential of said anode on theelectrostatic field between said oscillator electrodes. Such means maycomprise a grid electrode surrounding or partly surrounding the spacebetween the electrodes, and serving to screen this space from the anode,and means for maintaining said grid at a substantially constantpotential positive with respect to the mean potential of said oscillatorelectrodes.

The invention in one embodiment will now be described with reference toFig. 2 of the accompanying diagrammatic drawings.

Referring to Fig. 2 a discharge device comprises an evacuatedcylindrical glass envelope I having disc shaped oscillator electrodes 2,3 one electrode being mounted near each end of the envelope I.Surrounding the space between the oscillator electrodes 2, 3 is a gridelectrode 4 in the form of a cylinder; the grid may be made from a helixof wire round suitable supporting rods (not shown). Co-axial with thegrid is arranged a tubular anode 5, which may be a sep arate metalcylinder as shown or may be in the form of a metallic deposit on theinside walls of the envelope I. Between the two oscillator electrodes 2,3, and externally of the envelope I is connected a parallel resonantcircuit comprising a condenser 6 and an inductance "I, and means, suchfor example as a coil 8 coupled electromagnetically to the inductance I,are provided for coupling a thermionic valve oscillator showndiagrammatically at 9 to the resonant circuit 6, 1. The oscillationsproduced by the oscillator 9 may have a frequency of the order of 50megacycles per second and be periodically stopped nd.

The tubular anode is connected through an output resistance It to thepositive terminal of a source of potential II, the negative terminal ofwhich is connected to the electrical centre of the inductance I. Thegrid electrode 4 is directly connected to a point on the source ofpotential I I which is less positive than that to which the anode 5 isconnected. One or both of the oscillator electrodes 2, 3 are coated withphoto-electrically active material I3, and the electrodes may be treatedin any known or suitable manner to increase the number of secondaryelectrons emitted.

A coil I2 carrying a direct current supplied from a suitable source notshown surrounds the envelope I and serves to produce a magnetic field inthe neighborhood of the anode 5 and parallel to the axis of the latter.

In operation, light of varying intensity is allowed to fall on one orboth of the oscillator electrodes 2, 3. In order to allow the light tofall on the oscillator electrodes one of these electrodes or the anode 5may be provided with a suitable aperture, or may have a length which isshorter than the distance between the oscillator electrodes 2, 3. In thecase where the anode 5 is in the form of a metallic deposit on the wallsof the envelope I, a part or the whole of the deposit may be made sothin as to be substantially transparent, to allow light to fall on theoscillator electrodes.

As already described, the electrons emitted from the illuminatedelectrode are oscillated to and fro between the electrodes 2, 3, and areincreased in number at each impact on an electrode by the phenomenon ofsecondary emission, so that a cloud of electrons oscillating between theoscillator electrodes 2, 3 is rapidly produced.

The grid electrode 4, being held at a positive potential with respect tothe oscillator electrodes 2, 3 draws oil a certain fraction of theelectron cloud. The electrons drawn off will pass through theinterstices of the grid 4 and will be attracted towards and be collectedby the anode 5, which is at a higher positive potential than the grid 4.This current in the anode circuit will set up potential difierencesacross the output resistance I0.

Now since there is no resistance in the circuit associated with the grid4, it will remain at a constant potential, which may beadjusted to thevalue at which the device operates most favourably. The current flowingin the anode circuit will cause the potential of the anode 5 to drop ata frequency of 5 megacycles per seca from the value when the oscillatorelectrodes are not illuminated, but since the anode 5 does not exert thewithdrawing effect on the electron cloud, this change of potential willnot afiect the stability of the device.

If desired a second grid electrode 14 may be inserted between the grid 4and the anode 5, and may be held at a potential less positive thaneither, for instance by connecting it to the negative terminal of thesource of potential. The purpose of this second grid is to preventsecondary emission from the anode, caused by the impact of the electronspassing through the grids, from entering the space between theoscillator electrodes.

The invention is not limited to the case where the grid or grids and theanode are tubular as may be seen by referring to Figure 3 of thedrawings. Furthermore, the invention is further not limited to the casewhere the primary electrons are photo-electrons liberated by light sincethe primary electrons may, for example, be derived from a heatedcathode, the emission from this cathode being controlled in any known orsuitable way, as by means of a control electrode the potential of whichis changed in accordance with the input signal to be amplified.

By referring to Fig. 3 it may be seen that the anodes 20 and 2| areshown as flat plates positioned in parallel relation. The grid or gridshave been omitted for clarity, but would, of course, be placed near eachof the anode plates and at the inside surface thereof. The corresponding grids as well as the anodes may be connected together, eitherinternally or externally. With this construction of the device, if theanode and grid or grids are considered as being on either side of thedevice, the light beam may be directed onto photo-sensitive cathodes, ifsuch cathodes are used, from above or below and hence no provision needbe made in the anode portions for allowing light to pass through theminto the interior of the device.

As suggested above, the cathodes may be heated to produce an electronemission, and in Figure 3 is shown a pair of cathodes 22 with filaments23 for heating the same. Current is supplied to the filaments 23 by theconductors 24 and to the cathodes by conductor 25. A control grid 26 isalso provided in such case to control the flow of electrons from thecathodes. It is to be understood, also, that in some cases only one ofthe two cathodes may be electrically heated and in such cases the othercathode may be only a metal plate for supplying secondary electrons orit may be photo-sensitized in order that the primary electrons may besupplied as a result of light or heat, alternatively.

In such a form as shown in Figure 3, the potentials of the grids andanodes would prevent excessive loss of primary or secondary electronsfrom between the anodes in a direction parallel to the plane of theanodes and furthermore externally or internally produced electrostaticor electromagnetic fields may be provided for this purpose.

It is understood that various modifications may be made in the inventionand it is desired that all such modifications be considered as fallingfairly within the spirit and scope of the appended claims.

We claim:

1. In an electron multiplying arrangement comprising an evacuatedenvelope, two oscillator electrodes arranged within said envelope inspaced relation, means for applying alternating potential difierencesbetween said electrodes for the purpose of causing electrons tooscillate between them, input means for varying the number of electronsoscillating between said oscillator electrodes, an anode substantiallysurrounding the space between said electrodes, and a circuit formaintaining said anode at a potential positive with respect to the meanpotential of the said oscillator electrodes, so that said anode isadapted to draw off some of said electrons, said circuit including anoutput impedance, and means for reducing the effect of changes ofpotential of said anode on the electrostatic field between saidoscillator electrodes.

2. An arrangement as claimed in claim 1 wherein the last mentioned meanscomprise a grid electrode substantially surrounding the space betweenthe electrodes, and serving to screen this space from the anode, andmeans for maintaining said grid at a substantially constant potentialpositive with respect to the mean potential of said oscillatorelectrodes.

3. An arrangement as claimed in claim 1 wherein at least one of saidoscillator electrodes are coated with photo-electrically activematerial, and said input means comprise a source of light adapted to bevaried in intensity.

l. An arrangement as claimed in claim 1 wherein the last mentioned meanscomprise a grid electrode substantially surrounding the space betweenthe electrodes, and serving to screen this space from the anode, andmeans for maintaining said grid at a substantially constant potentialpositive with respect to the mean po tential of said oscillatorelectrodes, and wherein at least one of said oscillator electrodes iscoated with photo-electrically active material, and said input meanscomprise a source of light adapted to be varied in intensity.

5. An arrangement as claimed in claim 1 wherein said input meanscomprise a source of electrons and control electrode means for varyingthe number of electrons proceedng from said source into the spacebetween said oscillator electrodes.

6. An arrangement as claimed in claim 1 wherein the last mentioned meanscomprise a grid electrode substantially surrounding the space betweenthe electrodes, and serving to screen this space from the anode, andmeans for maintaining said grid at a substantially constant potentialpositive with respect to the mean potential of said oscillatorelectrodes, and wherein said input means comprise a source of electronsand control electrode means for varying the number of electronsproceeding from said source into the space between said oscillatorelectrodes.

'7. An arrangement' as claimed in claim 1, wherein the last mentionedmeans comprise a grid electrode substantially surrounding the spacebetween the electrodes, and serving to screen this space from the anode,and means for maintaining said grid at a substantially constantpotential positive with respect to the mean potential of said oscillatorelectrodes, and wherein said anode comprises two portions in electricalconnection with one another and arranged one on each side of the spacebetween said oscillator electrodes and said grid comprises two portionsin electrical connection with one another and arranged between the twoportions of said anode and the space between said oscillator electrodes.

8. An ararrangement as claimed in claim 1 wherein the last mentionedmeans comprise a grid electrode substantially surrounding the spacebetween the electrodes, and serving to screen this space from the anode,and means for maintaining said grid at a substantially constantpotential positive with respect to the mean potential of said oscillatorelectrodes, and wherein between said anode and said grid there isprovided a second grid electrode, and means for maintaining said secondgrid at a potential which is lower than either of the potentialsapplied, in operation, to said anode and to said first-mentioned grid.

9. An arrangement as claimed in claim 1, wherein the last mentionedmeans comprise a grid electrode substantially surrounding the spacebetween the electrodes, and. serving to screen this space from theanode, and means for maintaining said grid at a substantially constantpotential positive with respect to the mean potential of said oscillatorelectrodes, and wherein said anode comprises two portions in electricalconnection with one another and arranged one on each side of the spacebetween said oscillator electrodes and said grid comprises two portionsin electrical connection with one another and arranged between the twoportions of said anode and the space between said oscillator electrodes,and wherein between said anode and said grid there is provided a secondgrid electrode, and means for maintaining said second grid at apotential which is lower than either of the potentials applied, inoperation, to said anode and to said first-mentioned grid.

10. An electron multiplying device comprising an evacuated envelope, aplanar thermionic cathode, a planar photoelectric cathode spaced fromand parallel to the thermionic cathode, a cylindrical anode surroundingsaid cathodes, a shielding electrode concentric with and positionedwithin the anode, and input means for controlling a number of electronsbetween the two cathodes.

HEINZ ERWIN KALLMANN. OTTO KLEM'PERER.

